Fluid catalytic cracking process with a segregated feed charged to the reactor

ABSTRACT

A hydrocarbon is cracked in the presence of a fluid zeolite catalyst or a catalyst of comparable activity which produces a transient maximum gasoline yield at a residence time of 5 seconds of less and in the presence of a diluent vapor or vapors which lower the partial pressure of the hydrocarbon feed or feeds and increase gasoline selectivity. Residence time is established by controlling the total charge rate of hydrocarbons and diluent vapors. The ratio of diluent vapors to hydrocarbon feed is also controlled so that a greater yield of gasoline is recovered from the process than could be recovered in the absence of the diluent vapor or Vapors. Gasoline yield is further enhanced by segregating the hydrocarbon feed and charging the relatively lower molecular weight feed fraction or fractions near the bottom of an elongated riser or transfer line rector and the relatively higher molecular weight feed fraction or fractions progressively further up the riser or transfer line.

United States Patent [72] Inventors Millard C. Bryson Conway, Pa.; JamesR. Murphy, Huntington Station, N.Y.

[21] Appl. No. 836,404

[22] Filed June 25,1969

[ 45] Patented Nov. 2, 1971 [73] Assignee Gulf Research & DevelopmentCompany Pittsburgh, Pa.

[54] FLUID CATALYTIC CRACKING PROCESS WITH A SEGREGATED FEED CHARGED TOTHE REACTOR 11 Claims, 4 Drawing Figs.

52 vs. C! 208/80, 23/288 s, 208/74, 208/120, 208/128, 208/130,

51 1111.01 ..C0lb33/28, Cl0gll/l8,Cl0qll/20 501 FieldofSearch..208/80,49, 128, 130, 153, 160; 23/288 [5 6] References Cited UNITEDSTATES PATENTS 10/1960 Marshall et al.

3,246,960 4/I966 Sharpetal 3,524,809 8/1970 Hansford 23/288 208/l I lABSTRACT: A hydrocarbon is cracked in the presence of a fluid zeolitecatalyst or a catalyst of comparable activity which produces a transientmaximum gasoline yield at a residence time of5 seconds ofless and in thepresence ofa diluent vapor or vapors which lower the partial pressure ofthe hydrocarbon feed or feeds and increase gasoline selectivity.Residence time is established by controlling the total charge rate ofhydrocarbons and diluent vapors. The ratio of diluent vapors tohydrocarbon feed is also controlled so that a greater yield of gasolineis recovered from the process than could be recovered in the absence ofthe diluent vapor or Vapors. Gasoline yield is further enhanced bysegregating the hydrocarbon feed and charging the relatively lowermolecular weight feed fraction or fractions near the bottom of anelongated riser or transfer line rector and the relatively highermolecular weight feed fraction or fractions progressively further up theriser or transfer line.

PATENTED'Mnv 2 l97| SHEET 1 IF 3 4 vy 0mm: ADDED r00 LATE REACTORRESIDENCE TIME 0 7'0 5 SECONDS ADD/770M I I l I 0 7'0 .5 SECONDS FROMT/MELDW MOLECULAR I'VE/6H7 CHARGE ADDED I N VENTORS M/LL 420 c. BRYSD/VJAMES R. MURPHY FLUID CATALYTIC CRACKING PROCESS WITH A SEGREGATED FEEDCHARGED TO THE REACTOR This invention relates to the cracking of apetroleum hydrocarbon feed stock to gasoline in the presence of a highlyactive fluid cracking catalyst such as a crystalline aluminosilicatezeolite or a catalyst of comparable activity, or selectivity, or both.

Natural or synthetic zeolite aluminosilicate cracking catalysts exhibithigh activity in the cracking of hydrocarbon oils both in terms of totalconversion of feed stock and in terms of selectively towards gasolineproduction. The present invention relates to a method for improving theselectivity to gasoline production in cracking processes utilizing afluidized zeolitic cracking catalyst or a catalyst of comparableactivity and/or selectivity.

In fluid catalytic cracking operations it is generally advantageous tooperate the cracking reactor at pressures in the range of about topounds per square inch gauge and it is undesirable in terms of theintegrated operation including catalyst regeneration and power recoveryfrom regenerator flue gases for reactor pressures to fall significantlybelow this level. For example, catalyst regeneration is generallyfavorably influenced by elevated temperatures and pressures.Furthermore, in systems where regenerator flue gas is utilized to drivea turbine to compress combustion air to be supplied to the regenerator,it is important to maintain an elevated pressure in the regenerator inorder to obtain efficient turbine operation. Since spent catalyst mustflow from the reactor zone to the regenerator, a correspondingly highpressure is consequently required in the reactor in order to urgecatalyst towards the regenerator. However, as shown below, relativelyhigh reactor hydrocarbon feed pressures are less favorable to gasolineselectivity in the cracking operation than relatively low pressures.

In accordance with the present invention a method is presented foradvantageously improving operation of a reaction process employing azeolitic or similar fluidized cracking catalyst without lowering thepressure in the reaction zone or catalyst disengaging or strippingvessel. We have discovered that an unexpected advantage occurs bycharging a diluent gas to the inlet of the cracking reaction zone tolower the partial pressure of the charge hydrocarbon in the reactionzone without disturbing the total pressure in the system. Any diluentwhich is a vapor or becomes a vapor under the condi-' tions of thereaction zone can be used. An inert gas such as steam or nitrogen is asuitable diluent. A mixture of gases can be employed. if the diluent isa hydrocarbon, it should desirably have a boiling point below about 430F., i.e. it should be a gasoline range hydrocarbon or lighter. If itboils above the gasoline range it will itself be a portion of thecracking feed. Recycle methane or ethylene could be employed. We havefound that a lower hydrocarbon feed partial pressure at any givenreaction zone total pressure produces the unexpected effect ofincreasing the selectivity to gasoline production at a given conversionlevel of fresh feed or, conversely, requiring a lower conversion oftotal feed to produce a given gasoline yield.

Although it has been known that the use of an inert diluent such assteam at the hydrocarbon feed zone accomplishes certain advantageouseffects in a fluid catalytic cracking operation such as assisting influidization of catalyst, vaporization of liquid feed, dispersal ofcatalyst into hydrocarbon feed, increasing reaction rate, etc., theimprovement in gasoline selectivity has not heretofore been appreciated.We have further discovered that the gasoline selectivity advantage istransient and is lost if the cracking process is not terminated in atimely manner, as explained below. Because of its transient nature theselectivity advantage has heretofore been effectively masked.

It has previously been considered that the amount of steam to beemployed in a fluid catalytic cracking process should not be great inorder to avoid a reduction in residence time, and thereby a loss inconversion. However, in accordance with the present invention the amountof steam or other inert gas must be sufficient to produce a significantreduction in partial pressure of the incoming hydrocarbon capable ofbeing cracked to gasoline. Although the initial increments of partialpressure reduction exert a greater effect upon gasoline selectivity thanlater increments, the greater the amount of steam or other inert gasintroduced relative to hydrocarbon feed the greater will be the effectupon selectivity. For example, 10 mol percent steam based on hydrocarboncharge will reduce the partial pressure of the hydrocarbon charge l0percent, 15 mol percent steam will reduce the partial pressure of thehydrocarbon charge 15 percent, etc., and the greater the reduction inpartial pressure the greater the gasoline selectivity advantage it ispossible to achieve in accordance with this invention.

In accordance with the present invention it has further been discoveredthat the selectivity advantage due to the presence of an inert gas,which is not itself capable of being cracked to gasoline, is mostsignificant in the very early stages of the cracking reaction, which isalso the period in which most of the cracking of fresh feed occurs. lnfact, the curve of production of cracked hydrocarbon vapors from freshfeed with time is exponential with the greatest rate of crackingoccurring at the outset of the reaction so that the cracked vaporsthemselves quickly reduce the partial pressure of the unreacted feed.However, by the time these vapors are produced most of the cracking hasbeen completed. The extent of cracking of fresh hydrocarbon feed with azeolite catalyst is considerably greater in the first 0.l secondinterval in the reaction zone than in the second 0.] second interval.Similarly, the extent of cracking of fresh hydrocarbon charge isconsiderably greater in the first 0.2 second interval in the reactionzone than in the second 0.2 second interval. For example, after thehydrocarbon feed has been in the reaction zone for about 0.1 second itis about 40 percent converted and after about 1.0 second conversionincreases only to about 70 to percent.

In control methods for fluid catalystic cracking operations according tothe prior art, a vapor such as steam was added to the inlet of anelongated riser or reaction zone to assist dispersal of catalyst intohydrocarbon. The amount of steam was not considered particularlycritical. Reactor residence time (space velocity) was then adjusted tocontrol gasoline yield in the reactor efiluent. If analysis of reactoreffluent indicated an adjustment of the residence time was required, thehydrocarbon flow rate was adjusted. But no criticality was attached tothe fact that this adjustment varied the ratio of steam to hydrocarbonat the reaction zone inlet. In accordance with the present inventionreaction zone residence time is established not only by establishing thetotal charge rate including both hydrocarbon and steam but also byestablishing the ratio of steam to hydrocarbon in the charge in themanner. described below. We have now discovered and it is shown belowthat control of the ratio of steam to hydrocarbon in the charge andcontrol of the total charge rate including both steam and hydrocarbonare interdependent and interdependently exert a critical effect ongasoline yield.

Although zeolitic aluminosilicates are especially useful catalysts forpurposes of the present invention, any silica alumina or other crackingcatalyst which is sufficiently active and/or selective to be capable ofproducing a transient maximum or peak gasoline yield from the totalfresh hydrocarbon feed capable of being cracked to gasoline at residencetimes of 5 seconds or less are within the purview of this invention. Themaximum gasoline yield obtained at residence times within 5 seconds istransient and rapidly diminishes. After a residence time of 1 second,most of the fresh hydrocarbon feed is converted and there is a sharpdrop in rate of conversion of fresh feed. However, if the hydrocarboncontinues to remain in contact with the catalyst, products of theearlier cracking operation themselves in turn undergo cracking. Thisoccurrence is termed aftercracking." Since there is a greater abundanceof cracked material than uncracked material after only about one-half to1 second of reaction zone residence time or less the situation rapidlyarises wherein considerably more cracking of cracked than uncrackedmaterial can occur.

When this situation prevails, the desired gasoline product initiallyproduced at a high selectivity in accordance with the present inventionbecomes depleted due to aftercracking at a faster rate than it isreplenished due to cracking of remaining uncracked feed so that theselectivity advantage initially achieved is subsequently lost at asignificant rate. lf timely disengagement of hydrocarbon and catalystdoes not occur prior to the occurrence of a significant amount ofaftercracking the very existence of the earlier advantageous selectivityeffect can be entirely masked. This invention requires substantiallyinstantaneous disengagement of catalyst and hydrocarbon as thesematerials exit from the reaction zone into a disengaging vessel.

In accordance with the present invention a preheated liquid hydrocarboncharge and a fluid zeolite or comparable cracking catalyst is added to acracking reaction zone together with an inert gaseous diluent such assteam, nitrogen, recycle methane or ethylene, etc. The liquidhydrocarbon charge is substantially instantaneously vaporized and thequantity of inert diluent is sufficient to accomplish a substantialreduction in the partial pressure of the hydrocarbon charge. Theselectivity to gasoline production is enhanced due to the lowerhydrocarbon partial pressure at the onset of cracking of the fresh feeddue to the presence of the diluent. In order not to subsequently losethe selectivity advantage the hydrocarbon is permitted to remain in thepresence of the catalyst only as long as further conversion of uncrackedhydrocarbon produces a significant increase in gasoline yield. Thesystem is controlled so that substantially at the time when furtherconversion of uncracked hydrocarbon produces no significant net increasein gasoline yield or at the time when some decrease in gasoline yieldensues the catalyst and hydrocarbon are substantially instantaneouslydisengaged from each other to prevent aftercracking of gasoline productfrom destroying the selectivity advantage initially achieved due to thediluent partial pressure effect. Analysis of the product to measuretotal conversion of fresh feed or gasoline yield or both will aid incontrolling the reactor in accordance with this invention. Theseanalyses will provide a measure of gasoline selectivity for controllingthe reactor. Reaction time duration can be adjusted by regulation oftotal feed rate, including hydrocarbon and steam, where the reactorheight is fixed.

In accordance with this invention, the reactor is operated so that thereis a continual increase in gasoline throughout substantially the entirelength of the reactor coupled with a decrease in fresh feed, which meansthat the reaction is terminated'at or near the time of maximum gasolineyield. There is a substantial absence of backmixing in the reactor sincethis would be conducive to aftercracking. Backmixing can be caused by anexcessive linear velocity which gives rise to turbulence or by theformation of a dense catalyst bed which induces turbulence in flowingvapors. The hydrocarbon remains in the reactor only until a decrease infresh feed content is not accompanied by any substantial further netincrease in gasoline. Maximum gasoline yield is accompanied by max imumgasoline selectivity.

The overall time of contact between hydrocarbon and catalyst can be aslow as about 0.5 second or less but not greater than about 5 seconds andwill depend upon many variables in a particular process such as theboiling range of the charge, the particular catalyst, the amount ofcarbon 0n the regenerated catalyst, the catalyst activity, the reactionzone temperature, the polynuclear aromatic content of the hydrocarbonfeed, etc. Some of these variables can affect one another. For example,if the fresh hydrocarbon charge includes a considerable quantity ofpolynuclear aromatics, the reaction should be permitted to proceed longenough to crack any monoor di-aromatics or naphthenes because thesecompounds produce relatively high gasoline yields and are the mostreadily crackable aromatics but the reaction should be terminated beforesignificant cracking of other polynuclear aromatics occurs becausecracking of these latter compounds occurs at a slower rate and resultsin excessive deposition of carbon on the catalyst. It is clear, that nofixed cracking time duration can be set forth but the time will have tobe chosen with the range of this invention depending upon the particularsystem. In one system, slightly exceeding a l.0 second residence timemight result in such severe aftercracking that the selectivity advantagewould be lost while in another system unless a 1.0 second residence timeis appreciably exceeded there might not be sufficient cracking of chargehydrocarbon to render the process economic. Generally, the residencetime will not exceed 2.5 or 3 seconds and 4 second residence times willbe rare.

Reference to FIG. 1 will illustrate the significance of the presentinvention. FIG. 1 contains curves semiquantitatively relating the amountof unreacted charge and gasoline, as percent based on fresh feed, toreaction zone residence time. The curve of unreacted charge which istypical of most fluid cracking charge stocks shows that the amount ofunreacted charge asymptotically approaches a value somewhat less than 20percent of fresh feed within residence times of this invention. Thecurves showing quantity of gasoline produced show that the quantity ofgasoline produced rapidly reaches a somewhat flat maximum or peak whichgenerally coincides with the time at which the cracking of unreactedcharge is substantially diminished. The gasoline yield at the peak for agiven feed will be determined mostly by reactor temperature, to anextent by the level of carbon on the catalyst and to an extent by thecatalyst to oil ratio. After reaching a peak, the gasoline leveldiminishes because the aftercracking of gasoline predominates overproduction of gasoline from the unreacted feed. The lower of the twogasoline curves shown in FIG. 1 indicates the level of gasoline in thereaction zone assuming substantially no inert diluent such as steam isintroduced to the inlet zone of the reactor. The upper of the twogasoline curves schematically shows the higher gasoline level achievedby adding an inert diluent such as steam to the inlet of the reactionzone which lowers the hydrocarbon feed partial pressure and therebyincreases selectivity to gasoline.

Assuming a fluid cracking process is operating with steam addition andthe gasoline yield is at point A shown in FIG. 1 where significantaftercracking has occurred. In order to reduce the extent ofaftercracking it is decided to increase the charge rate of hydrocarboninto the reaction zone, thereby reducing hydrocarbon residence time.Residence time is usually adjusted by adjustment of hydrocarbon chargerate rather than steam charge rate since for any given percentageincrease or decrease in charge rate of steam or hydrocarbon, the effectupon reaction residence time will be much greater in the case of thehydrocarbon adjustment because the total amount of hydrocarbon chargedis so much greater than the total amount of steam charged. Due to theshorter residence time and concomitant reduction in aftercracking ahigher gasoline yield B is achieved. However because the hydrocarbonpartial pressure at the reaction zone inlet has been increased by anincrease in hydrocarbon flow rate, the point B is removed from the uppergasoline curve in the direction of the lower gasoline curve and isoutside the cross-hatched zone which denotes the range of thisinvention. The crosshatched zone shown in FIG. 1 denotes the transientelevated gasoline yields of this invention which can be recovered by theuse of an inert vapor but which could not be recovered absent an inertvapor. On the other hand, if the same 1 decrease in hydrocarbonresidence time were achieved by increasing both hydrocarbon and steamflow rate in the same ratio so that the partial pressure of hydrocarbonat the reaction zone inlet remained unchanged at the new residence time,the new operating point would be at B, instead of B, which is within therange of the present invention. (Of course, if the same total flow ratewere achieved by increasing the ratio of steam to hydrocarbon the newoperating point would be above B and the area covered by thecross-hatched zone of this invention would be enlarged.) Now, if thehydrocarbon charge rate is again increased to further reduce residencetime, the point C is reached which is further removed from the uppergasoline curve in the direction of the lower gasoline curve than ispoint B because the hydrocarbon partial pressure is further increased ingoing from point B to point C. Again, because of the increase inhydrocarbon partial pressure, point C is outside the range of theinvention. On the other hand, if the same residence time indicated atpoint C is achieved by increasing the flow rate of both steam andhydrocarbon, rather than hydrocarbon alone, so that the hydrocarbonpartial pressure at the new residence time is the same as it was atpoint A, the point C is achieved which is within the range of thisinvention.

It is seen from FIG. I, that operating points B and C representessentially similar gasoline conversion levels occurring at differentresidence times apparently indicating that these points lie close to aflat maximum gasoline yield. However, points B and C lie outside therange of the present invention while operating points B and C, which arewithin the range of this invention, lie at higher gasoline yield levelsthan points B and C, even through points B and B and points C and Crepresent the same residence times, respectively. Starting from point A,point B is reached by the method of lowering residence time via a changein both steam flow rate and hydrocarbon flow rate while, also startingfrom point A, point B is reached by the method of changing hydrocarbonflow rate only to achieve the same residence time as point B. Startingfrom point B, point C is reached by changing both steam flow rate andhydrocarbon flow range to lower the residence time, while point C isreached by the simpler method of changing hydrocarbon flow rate only toachieve the same residence time as at point C. it is apparent that toachieve the gasoline selectivity advantage of the present invention, theresidence time and the apportioning of steam and hydrocarbon flow ratesto achieve said residence time are interdependent and represent acritical combination for purposes of process control.

While the partial pressure effect of this invention tends to increaseselectivity to gasoline, there is a competing effect in a crackingprocess which tends to oppose and thereby mask the partial pressureeffect. This competing effect arises due to carbon laydown on thecatalyst as the catalyst travels through the reaction zone. As theamount of carbon on the catalyst increases along the reaction path thegasoline selectivity from the feed decreases. The higher the molecularweight of the feed hydrocarbon the greater the carbon on catalystcompeting effect because the high molecular weight components tend tocontain more polynuclear aromatic compounds which yield more coke oncracking than other compounds. Of the aromatic compounds, thepolynuclear compounds not only crack at a slower rate but also have amuch higher selectivity to C and lighter gases and coke, while themonoand di-aromatics and the alkyl side chains of naphthenes tend notonly to crack at a faster rate but also to exhibit a higher selectivityto gasoline. Therefore, the heavier hydrocarbon feed com ponents shouldbe subjected to a reduced residence time, such as only about 0.5 to 1.5seconds, in order to limit the cracking thereof as much as possible toparaffinic side chains and monoand di-aromatics in general.

In accordance with this invention, the feed hydrocarbon in fractionatedand a fraction containing the relatively lower molecular weightcomponents (predominantly paraffms, naphthenes and monoand di-aromatics)to be cracked is charged together with catalyst to the bottom of anelongated reaction zone and permitted to undergo substantial crackingbefore reaching the position in the reaction path of entry of a fractioncontaining the relatively higher molecular weight components (whichcontain more predominantly the polynuclear aromatics). After the majorportion of the cracking of the lower molecular weight fraction hasoccurred, the higher molecular weight fraction is introduced to thereactor without additional catalyst. in this manner most of the lighterhydrocarbon feed is cracked in the absence of the heavy hydrocarbon feedand thus on a low carbon content catalyst. The cracking operation forthe lower molecular weight feed is optimized (maximum gasolineselectivity) under the combined influence of the reduced partialpressure effect of the inert diluent, low carbon on catalyst efi'ect,and a somewhat more severe cracking operation (i.e. high catalyst to oilratio). The heavy hydrocarbon feed is then subjected to a much shorterresidence time than the lighter feed. If desired, one or more relativelylight hydrocarbon feed streams can be introduced near the bottom of thereactor and one or more relatively heavy hydrocarbon feed streamsderived from the same or a different source than that from which thelight feed is derived can be introduced relatively downstream along thereaction path, the heavier the feed (or more polynuclear aromatic) thefurther downstream its position of introduction. A heavy charge streamcan comprise recycle in whole or in part. At each position ofintroduction of heavy feed, the diameter of the reactor can increase sothat the velocity at the inlet of the reactor and at the outlet of thereactor will be about the same. If desired, the reactor can be taperedto provide increasing diameters along the reaction path to provide auniform velocity throughout. A high degree of control in the reactor isachieved by varying the amount and position of introduction of theheavier hydrocarbon feed or feeds relative to the amount and position ofthe lighter feed or feeds in order to vary the residence time of allmaterial flowing through the reactor. in accordance with this invention,the downstream position of introduction of the high molecular weighthydrocarbon feed stream is established so that a greater per centageyield of gasoline from the high molecular weight feed is recovered fromthe process in the presence of the low molecular weight reactionproducts stream (partial pressure effect) than could be recovered in theabsence of the low molecular weight reaction products stream.

It is shown below that segregating the total hydrocarbon feed intorelatively high and low molecular weight fractions as described providesan increased selectivity to gasoline as compared to charging the fullrange hydrocarbon feed to a single position at the bottom of thereactor. Data presented below indicates that a heavy carbon laydown onthe catalyst (such as is contributed by heavier feeds) is a greaterdetriment to gasoline selectivity when cracking a relatively low boilingfeed than when cracking a relatively high boiling feed, although it is adetriment with both. Therefore, a net advantage in terms of gasolineselectivity is achieved by permitting the low molecular weight feed toundergo most of its cracking in the absence of the heavy feed and thuswith a catalyst having a low level of carbon. Thereupon, when the heavyfeed stream is introduced at a position downstream along the reactionpath, the reaction products of the lighter feed serve to lower thepartial pressure of the heavy feed stream to a great extent, which inturn tends to offset the gasoline selectivity disadvantage the heavyfeed experiences due to being cracked in the presence of a used andunregenerated carbon-containing catalyst. It is seen that the carbon oncatalyst effect and the vapor pressure effect arising due to employing asegregated feed as described constitute interdependent efiects whichcooperate to enhance gasoline selectivity in the over-all process.

FIG. 2 illustrates the control method of this invention for a reactorwherein a relatively low molecular weight hydrocarbon fraction is addedto the bottom of the reactor and a relatively high molecular weighthydrocarbon fraction is charged to the reactor at a position above thebottom of the reactor and downstream along the reaction path from theposition of entry of the low molecular weight fraction. in anadvantageous embodiment a full range feed is fractionated to segregateit into two fractions and the lower molecular weight fraction is chargedto the bottom of the reactor while the higher molecular weight fractionis charged to a higher position in the reactor. The two fractions can beequal or unequal in volume. Substantial cracking (but not the optimum)of the low molecular weight fraction occurs in advance of the positionof charging of the high molecular weight fraction.

As shown in FIG. 2, the curved dashed lines indicate the gasoline yieldat varying residence times for the relatively light charge, the lowercurved dashed line indicating gasoline yield without added vapor and theupper curved dashed line indicating gasoline yield with added vapor. Theenclosed unhatched region above the horizontal dashed line M indicatesthe additional gasoline yield achievable due to the use of a vapor withthe light charge because of the hydrocarbon partial pressure reductionat the reactor inlet. The vertical line X indicates the maximumallowable residence time if this additional gasoline yield is to beactually recoverable.

The dotted lines of FIG. 2 indicate the addition of the heavyhydrocarbon fraction at a position in the reactor which is so high thatthe heavy feed does not have time to reach a maximum gasoline yieldbefore reaching residence time line X.

The curved solid lines of FIG. 2 indicate the addition of the heavyhydrocarbon fraction at a position in the reactor which is above thebottom of the reactor but which is sufficiently close to the bottom thatthe heavy charge is in the reactor for a sufficiently long time durationto achieve a maximum gasoline yield. The unhatched enclosed area abovethe horizontal solid line N represents the additional gasoline yieldachievable from the heavy charge due to the presence of the vapors fromthe reaction stream derived from the light charge which lower thepartial pressure of the heavy hydrocarbon feed at the position ofadmission of said heavy hydrocarbon. The vertical line Y demarcates thelowest residence time permissible if this additional gasoline yieldobtainable from the heavy charge is to be actually recoverable.

It is seen from FIG. 2 that the residence time interval bracketed byvertical lines X and Y is the only interval in which the additionalgasoline yield derived from both the light and heavy charge due to vaporpressure reduction in the feed zone of each can actually be recoveredfrom the reactor. Therefore, the position denoted by B which wasdiscussed above' in regard to FIG. 1 lies between lines X and Y and alsolies above dashed horizontal. line M with which it is associated so thatat this position the additional gasoline yield obtainable from both thelight and the heavy charge can be recovered from the process. On theother hand, the position denoted by C which was also discussed above inregard to FIG. 1 lies outside the bracket established by the lines X andY so that while the additional gasoline yield from the light charge isrecoverable the additional gasoline yield from the heavy charge is notrecoverable. Therefore, the residence time corresponding to the point Cin FIG. 2 is not suitable when charging a segregated feed in accordancewith the present invention.

It will be evident that only those gasoline yield points in FIG. 2 lyingbetween lines X and Y which also lie above the dashed or solidhorizontal line M or N with which they are associated fall within thepurview of this invention. For example, the position B in FIG. 2, whichposition was also discussed above in regard to FIG. 1, although it fallsbetween residence time lines X and Y it does not fall above horizontaldashed line M with which it is associated, and therefore lies outsidethe purview of this invention. Therefore, the positions B and C whichfall outside the limits of this invention in accordance with thediscussion regarding the single hydrocarbon feed system of FIG. 1,remain outside the confines of this invention according to the dual feedsystem illustrated in FIG. 2.

In any particular process the gasoline yield and residence time valueswhich encompass the gasoline selectivity advantage of the presentinvention will depend upon many variables peculiar to the particularprocess. These variables include the particular catalyst which isemployed, the level of carbon on the regenerated catalyst, catalystactivity and/or selectivity, the temperature, the refractorycharacteristics of the feed, etc. The extent of this selectivityadvantage of this invention might be as low as one-half percent to 1percent or as high as 3 to 5 percent depending upon the ratio of diluentvapor to hydrocarbon feed at the reactor inlet and the apportionment ofcharges and their respective feed locations. Where gasoline is the mosteconomically desirable product of the cracking operation, the economicvalue of a selectivity advantage of even one-half or 1 percent actuallyrecovered as effluent is considerable in a commercial reactor unit whichprocesses 100,000 or 150,000 barrels per day of hydrocarbon feed.

The reaction temperature in accordance with this invention is at leastabout 900 F. The upper limit can be about l,l00 F., or more. Thepreferred temperature range is 950 to l,050 F. The reaction totalpressure can vary widely and can be, for example, 5 to 50 p.s.i.g., or,preferably, 20 to 30 p.s.i.g. The maximum residence time is 5 seconds,and for most charge stocks the residence time will be about 1.5 secondsor 2.5 seconds or, less commonly, 3 or 4 seconds. For high molecularweight charge stocks which are rich in aromatics a 0.5- to 1.5- secondresidence time could be suitable in order to crack monoand di-aromaticsand naphthenes which are the aromatics which crack most easily and whichproduce the highest gasoline yield, but to terminate the operationbefore appreciable cracking of polyaromatics occurs because thesematerials produce high yields of coke and C, and lighter gases. Thelength to diameter ratio of the reactor can vary widely, but the reactorshould be'elongated to provide a high linear velocity, such as 25 to 75feet per second, and to this end a length to diameter ratio above 20 or25 is suitable. The reactor can have a uniform diameter or can beprovided with a continuous taper or a stepwise increase in diameteralong the reaction path to maintain a nearly constant velocity along theflow path. The amount of diluent can vary depending upon the rate ofhydrocarbon to diluent desired for control purposes. If steam is thediluent employed, a typical amount to be charged can be about 10 percentby volume, which is about 1 percent by weight, based on hydrocarboncharge. A suitable but nonlimiting proportion of diluent gas, such assteam or nitrogen, to fresh hydrocarbon feed can be 0.5 to 15 percent byweight.

A zeolite catalyst is a highly suitable catalytic material in accordancewith this invention. A mixture of natural and synthetic zeolites can beemployed. Also a mixture of crystalline zeolitic organosilicates withnonzeolitic amorphous silica aluminas is suitable as a catalytic entity.Any catalyst containing zeolitic material or otherwise which provides atransient maximum gasoline yield within a S-second residence time issuitable. The catalyst particle size must render it capable offluidization as a disperse phase in the reactor. Typical and nonlimitingfluid catalyst particle size characteristics are as follows:

o-zo 454s 75 Size (Microns) 20-45 Weight percent 0-5 20-30 35-55 20-40These particle sizes are usual and are not peculiar to this invention. Asuitable weight ratio of catalyst to total oil charge is about 4:1 toabout l2zl or 15:1 or even 25:1, generally, or 6:1 to 10:1, preferably.The fresh hydrocarbon feed is generally preheated to a temperature ofabout 600 to 700 F. but is generally not vaporized during preheat, andthe additional heat required to achieve the desired reactor temperatureis imparted by hot, regenerated catalyst.

The weight ratio of catalyst to hydrocarbon in the feed is varied toaffect variations in reactor temperature. Furthermore, the higher thetemperature of the regenerated catalyst the less catalyst is required toachieve a given reaction temperature. Therefore, a high regeneratedcatalyst temperature will permit the very low reactor density level setforth below and thereby help to avoid backmixing in the reactor.Generally, catalyst regeneration can occur at an elevated temperature ofabout 1,240 F. or 1,250 F. or more to reduce the level of carbon on theregenerated catalyst from about 0.6 to 1.5 to about 0.05 to 0.3 percentby weight. At usual catalyst to oil ratios in the feed, the quantity ofcatalyst is more than ample to achieve the desired catalytic efi'ect andtherefore if the temperature of the catalyst is high, the ratio can besafely decreased without impairing conversion. Since zeolitic catalystsare particularly sensitive to the carbon level on the catalyst,regeneration advantageously occurs at elevated temperatures in order tolower the carbon level on the catalyst to the stated range or lower.Moreover, since a prime function of the catalyst is to contribute heatto the reactor, for any given desired reactor temperature the higher thetemperature of the catalyst charge the less catalyst is required. Thelower the catalyst charge rate the lower the density of the material inthe reactor. As sated, low reactor densities help to avoid backmixmg.

The reactor linear velocity, while not being so high that it inducesturbulence and excessive backmixing, must be sufficiently high thatsubstantially no catalyst accumulation or buildup occurs in the reactorbecause such accumulation itself leads to bacltmixing. (Therefore, thecatalyst to oil weight ratio at any position throughout the reactor isabout the same as the catalyst to oil weight ratio in the charge.)Stated another way, catalyst and hydrocarbon at any linear positionalong the reaction path both flow concurrently at about the same linearvelocity, thereby avoiding significant slippage of catalyst relative tohydrocarbon. A buildup of catalyst in the reactor leads to a dense bedand backmixing which in turn increases the residence time in the reactorfor at least a portion of the charge hydrocarbon and inducesaftercracking. Avoiding a catalyst buildup in the reactor results in avery low catalyst inventory in the reactor, which in turn results in ahigh space velocity. Therefore, a space velocity of over 100 or 120weight of hydrocarbon per hour per weight of catalyst inventory ishighly desirable. The space velocity should not be below 35 and can beas high as 500. Due to the low catalyst inventory and low charge ratioof catalyst to hydrocarbon, the density of the material at the inlet ofthe reactor in the zone where the low molecular weight feed is chargedcan be only about 1 to less than 5 pounds per cubic foot, although theseranges are nonlimiting. An inlet density in the zone where the lowmolecular weight feed and catalyst is charged below 4 or 4.5 pounds percubic foot is desirable since this density range is too low to encompassdense bed systems which induce backmixing. Although, conversion fallsoff with a decrease in inlet density to very low levels, we have foundthe extent of attercracking to be a more limiting feature than totalconversion of fresh feed, even at an inlet density of less than 4 poundsper cubic foot. At the outlet of the reactor the density will be abouthalf of the density at the inlet because the cracking operation producesabout a fourfold increase in mols of hydrocarbon. The decrease indensity through the reactor can be a measure of conversion.

A wide variety of hydrocarbon oil charge stocks can be employed. Asuitable charge is a gas oil boiling in the range of 430 to l,100 F. Asmuch as 5 to 20 percent of the fresh charge can boil above this range.Some residual oil can be charged. A to percent recycle rate can beemployed. Generally, the recycle will comprise 650 F. oil from theproduct distillation zone which contains catalyst slurry. If there is nocatalyst entrainment, recycle can be omitted.

EXAMPLE 1 A series of tests were conducted which illustrate the effectof reducing hydrocarbon partial pressure upon selectivity to debutanizedgasoline and to C -Hiquid yield. The tests were conducted in anelongated reactor and the hydrocarbon partial pressure was reduced byaddition of steam and nitrogen with the feed hydrocarbon. The ranges ofconditions of the various tests were as follows:

Charge Stock Inspections Zcolite (50-60 Kellogg 2 Hour Activity)Catalyst Cracking Conditions Temperature: "F. 950 Contact Time: Seconds0.1-2.0 Cat-tO-Oil Ratio 6.5-9.0 Recycle none Riser Total Pressure:

p.s.i.g. 23-30 Riser Gal Comporition (Inlet):

Mol Percent Hydrocarbon 5-80 Steam 5-90 Nitrogen 2-31 The results of thetests are illustrated in FIG. 3 in which debutanized gasoline yield andtotal C;,+liquid yield, both reported as percent by volume of freshfeed, are plotted against total conversion at various partial pressuresof hydrocarbon in the system and at various residence times. Thepressure ranges given on the face of the graphs indicate the partialpressure in the system of all hydrocarbon vapors, cracked and uncracked,with the remainder of the reactor pressure accounted for by nitrogen andsteam, both nitrogen and steam being used in all tests. For each partialpressure, conversion data is indicated for one or more residence times.

As shown in FIG. 3, at any given conversion level the selectivity togasoline as well as to total C -Hiquid increases with decreasinghydrocarbon partial pressure. Taking a 60 percent conversion level forpurposes of example, when the hydrocarbon partial pressure is 16-20p.s.i.g., the gasoline yield is 47.5 percent; when the hydrocarbonpartial pressure is 10-14 p.s.i.g. the gasoline yield increases toalmost 50 percent; and when the hydrocarbon partial pressure is 2-5p.s.i.g. the gasoline yield increases still further to about 51.5percent. Advantageously, a greater improvement in gasoline selectivityoccurred in reducing hydrocarbon partial pressure from 16-20 p.s.i.g. to10-14 p.s.i.g. than occurred in reducing hydrocarbon partial pressurefrom 10-14 p.s.i.g. to the very low partial pressure level of 2-5p.s.i.g. This shows that the gasoline selectivity advantage of thisinvention was realized to a very significant extent in the initialpartial pressure reduction step of the tests and the effect was not asgreat but still substantial in the second partial pressure reductionstep of the tests.

EXAMPLE 2 Tests were conducted to illustrate the advantage of acrystalline zeolite aluminosilicate catalyst over an amorphoussilica-alumina catalyst in a fluid catalytic cracking system. Bothcatalysts were tested under sufficiently low space velocity conditionsthat a dense phase bed formed in the reactor. The results are shown intable 1 TABLE 1 Charge Stock Characterization Factor 12.09 I 1.95Gravity: AP1 29.7 29.4 Sulfur: Percent 0.42 0.36 Viscosity, SUS at: F.

210 38.6 37.3 Carbon Residue,

Ramsbottom: Percent ASTM Aniline Point; F. 188 184 Bromine Number. D11592.8 3.0 Pour Point, 1397: I 90 Nitrogen: p.p.m. 710 450 Metull: p.p.m.

Vanadium 0.2 0.4

Nickel 0.2 0.1 Distillation Vac. (Corres. to

760 mm. Hg)

10% over at: F. 568 556 95 to bed formation is permitted to occur. Theresults are shown in table 2.

Catalyst I percent 60 percent Amorphous silicazeolite, 40 aluminapercent TABLE 2 silicaalumina Test 1 2 3 4 Kellogg Activity Catal I) lst Catalyst bed formatiom. Yes No Yes No Cracking temperature. F 950 9501.000 1. 000 Space velocity ttotalteed) 19.2 100 19.3 100 Reacm' Contacttime, seconds 0. 5 (1) 2.0 Recycle. percent by volume 2. 4 5. 3 NoneNone Conversion. percent by volume. 72. 9 77.1 76. 2 80.11 Fresh FeedRate: BID 13.704 Yields. percent by volume of Reactor Bed Temperature:F. 926 935 fresh feed: Feed Preheat Temperature: F. 700 649 Total: R C19.0 10.4 11.7 .3 eactor Bed Pressure. p.s.1.g. ll.5 11.0 C 6 5 q 0 5 v1T 1F 4- l5 pace eoc1y.( ota ce Total.

Wt./Hr./Wt. 3.94 3.07 0. 14. 2 16. 0 15.8 17. 7 Catalyst to Oil Ratio 4=6. 8 7. 6 8. 0 7. R

(Tomi Feed); w w 12,5 93 Debutanized gasoline 545. 8 5= 8 t .1 74 3 3| 407 plus gasolino 44.2 47.0 44.8 50.11 Total 0. plus liquid 106.8 100.5107. 5 111.1! Carbon on Rcgeneratcd Cat. C; and lighter. percent by k bWt. 04 0.38 weight 3. 6 .5 4.1 3.1 Conversion: '5 by Volume G ({pke.ptercent by weight, 5.6 .0 5. 0 4. 5

850 ill! 00 Bile: Motor. clear 711. 3 .6 so. a 711. 3 lklotor. rgnsliicc 85.?! .2 86.2 esearc .c ear 92. 6 9.. .Il. Operation Cond1t1ons.Regenerator Research p u 3 cc 10 2 3 99. 5 U8. 7

2mins. Regen. Bed Temperature: F. l,l4l 1,166 3 Total Regen. Air:MlbJl-lr. 153.7 166.72 Bed backm'xmg' firm 0 087 0 083 A comparison oftests l and 2 of table 2, both conducted at 950 F., shows thedeleterious effect of extended residence Yields: by volumc of Fresh Feedtlme when employing a zeohte catalyst. The resldence time of test 2 wasonly 0.5 second and yet It exhlblted a higher Debmnmd Gasoline 4Hgasolme y1eld and a lower C1 and lighter yleld than test l in Bumwmne2L2 2L6 wh1ch the residence ume was cons derably longer due to ai-Butanc 7.6 10.3 lower space veloc1ty and backm1x1ng ar1s1ng 1n thedense B H catalyst bed. A comparison of tests l and 2 shows that an ex-Propylene tended residence time gives arise to aftercracking whichPropane 4.2 5.7 diminishes gasoline yield and increases the yield ofproducts P em boiling lower than gasoline. Zi'gh'tifgl 40 Comparing test3 with test l, both involving dense bed Z wL M 249 cracking, it is seenthat raising the cracking temperature from Coke: 91 by m. 7.73 7.11 950to L000 F., provided a slgmficant mcrease 1n convers1on but very littleincrease in debutanized gasoline yield and a Inspections higher yield ofC and lighter, showing that the high degree of aftercracking occurringin a dense bed reaction system MotonClcar 111.3 prevents effectivecontrol of gasoline yield via temperature Motor. +3 cc. TEL 86.l 119.4adjustment. Research Clear. 94.0 93.4

- o r s 4 w 5 v0 vin n nb d Search cc TEL 100A 983 C mpa mg te t 1th tet 2, both in l g o e As shown in tablev l, the zeolite catalyst systemexhibited a conversion of 85.5 percent compared to only 75.5 percent forthe amorphous catalyst. in addition, the zeolite catalyst systemexhibited a 6L0 percent yield of gasoline compared to only 47.5 percentgasoline yield with the amorphous catalyst. However, while the totalyield of C and C hydrocarbons is about the same for the zeolite and theamorphous catalyst, the proportion of these C and C hydrocarbons whichare oleflnic is lower when utilizing a zeolite catalyst in these tests.This is a disadvantage arising when utilizing a zeolite catalyst withextended residence times in a dense catalyst bed because C and C olet'msare useful for the production of alkylate which can be blended with thegasoline produced directly by cracking to improve its octane value.

EXAMPLE 3 Further tests were conducted to illustrate the use of the sametype of zeolite catalyst employed in example 2 for fluid catalyticcracking not only at relatively high residence times involving spacevelocities low enough to permit a dense phase catalyst bed to form inthe reactor but also at very low residence times within the range ofthis invention at which the velocity through the reactor is sufficientlyhigh that no bed formation within the reactor and therefore nobackmixing due cracking and very low residence times within the range ofthis invention, it is seen that raising the cracking temperature from950 to l,000 F. provided not only a significant increase in conversionbut also an equally significant increase in gasoline yield coupled witha lower yield of both C and lighter and coke, showing that thecomparative absence of aftercracking at the very low residence times ofthis invention permits control of gasoline yield via temperatureregulation. It is also noted that test 4provided good yields of C olefinand C olefin which are valuable materials for preparation of alkylategasoline.

Since table 2 indicates that in low residence time nondense bed systemsgasoline yield can be effectively controlled via temperature regulation,it follows that a reduction in temperature might be useful on occasionin an operating plant to reduce gasoline yield as required by subsequentfractionator load or to decrease C olefin and C olefin production.However, no matter what the operating temperature is the gasoline yieldat that temperature is increased by utilizing the control method of thisinvention.

EXAMPLE 4 Table 3 shows the results of four tests including a test basedupon calculation which illustrate the advantageous effect on gasolineyield achievable by fractionating a hydrocarbon cracking feed into arelatively high molecular weight fraction and a relatively low molecularweight fraction and separately cracking the fractions in the presence ofa zeolite catalyst. Test 1 of table 3 shows the results where a fullrange hydrocarbon feed is charged to the bottom of a single reactor.Test 3 shows the results where the total feed is fractionated and thelighter 50 percent by volume is alone charged to the bottom of a singlereactor. Test 4 shows the results where the heavier 50 percent by volumeof the fresh feed is alone charged to the bottom of a single reactor.Test 2 shows the calculated results of an integrated process wherein atotal hydrocarbon feed is segregated so that the lighter 50 percent byvolume is charged to one reactor and the heavier 50 percent by volume ischarged to another reactor and the effluents of the two reactors arecombined. All tests were made at a sufficiently low velocity that adense fluid catalyst bed was formed. All the tests were conducted at thesame hydrocarbon partial pressure at the reactor inlet.

TABLE 3 Test 1 2 3 4 Catalyst Charge stock Charge stock inspections:

Gravity, API 25. 6 25. 6 30. 5 21. 4 Sulfur, percent by weight 0. 8 0.80. 65 1. Ramsb. carbon residue, percent by weight 0. 42 0. 42 0. 09 0.73 Vacuum distillation (corres. to 760 mm. Hg) F. at, percent by volume:

10 580 580 510 809 30 692 692 629 831 50 767 767 659 873 70- 847 847 684921 90 969 969 712 1,016 C ,\-percentage of total atoms which arearomatic atoms 0. 18 0. 17 20 Operating conditions:

Temperature, F 940 940 940 .140 Space velocity (total feed) weight/hour/weight 6. 2 6.2 6. 2 6. 2 Gatalyst-to-oil ratio (total 1'eed) 7. 98. 0 8. 1 8. 0 Slurry oil recycle, percent by volume of fresh feed 5. 25.0 9. 1 Carbon on spent catalyst, percent by weight 1. 22 1.25 0.93 1.55 Carbon on regen. cat., percent by weight 0.3 0.3 0.3 0.3 Gas oilconversion, percent by volurne of fresh feed 80. 7 85. 2 Yields, percentby volume ol'Iresh feed:

Debutanized gasoline 60. 4 l Butane-butene 16. 2 17. 7 Isobutane 9. 1 8.3 n-Butane 2. 2 2. 1 Butenes 4. l 7. 3 Propane-propylene 10. 5 12. 1Propane 4. 8 5. 2 Propylene 5. 7 6. 9 Light catalyst gas 011. 19. 3Decantedoil- 11.9 Total 106. 4 103. 7

Gas, 02 and lighter, percent by weight 3 5 4. 4 Coke, percent by weight5. 6 12. 5

Total 9.1 16.9

HzS,percent by weight... 0.1 0.2

1 Zeolite-bed. 2 Full range unsegregated. 3 Segregated-calculated totalyield derived from both streams. 4 Light 50 percent by volume only offull range feed. 5 Heavy 50 percent by volume only of full range feed.

Comparing test 3 and test 1 of table 3, it is seen that cracking thelight charge alone resulted in about the same conversion as was obtainedwith a full range charge but at a significantly higher gasoline yield,indicating higher gasoline selectivity. Furthermore, the average carbonlevel on the catalyst in test 3 was 0.93 less 0.3, or only 0.63 percent,while the average carbon level on the catalyst in test 1 was 1.22 less0.3 or 0.92 percent. Again, the total C and lighter plus coke yield intest 3 was only 9.1 percent while the total C and lighter plus copeyield in test 1 was 12.6 percent. in all these respects the cracking ofthe light fraction by itself is superior to the cracking of a full rangecharge.

Opposite results are indicated by comparing test 4 with test 1, wherebyit is seen that cracking the heavy charge alone results in a much higherconversion than was obtained with a full range charge but at only aslightly higher gasoline yield, indicating much lower gasolineselectively. Furthermore, the average carbon level on the catalyst intest 4 was 1.55 less 0.3 or 1.25 percent, while the average carbon levelon the catalyst in test 1 was only 0.92 percent. Again, the total C andlighter plus coke yield in test 3 was 16.9 percent while the total C andlighter plus coke yield in test 1 was only 12.6 percent. In all theserespects the cracking of the heavy fraction by itself is inferior to thecracking of a full range charge.

Now, comparing calculated test 2 with test 1, it is seen that thecombined effects of tests 3 and 4 discussed above result in anintegrated process which is favorable to gasoline selectivity in thatgasoline yield is increased from 58.8 to 59.8 percent of fresh feed.Therefore, the segregation of the fresh field as described in this testresults in a higher gasoline yield and can cooperate with the vaporpressure effect described above in increasing gasoline yield with agiven hydrocarbon fresh feed.

A suitable reactor-regenerator system for performing this invention isdescribed in reference to FIG. 4. The cracking occurs with a fluidizedzeolitic catalyst in an elongated reactor tube 10, which is referred toas a riser. The riser has a length to diameter ratio of above 20, orabove 25. Hydrocarbon oil feed to be cracked in line 2 is firstfractionated in column 4 into a relatively low molecular weight fractionwhich flows through line 6 and a relatively high molecular weightfraction which flows through line 8. The low molecular weight fractionis passed through preheater 11 to heat it to about 600 F. and thencharged into the bottom of the riser through inlet line l4. Steam isintroduced into the low molecular weight oil inlet line through line 18.Steam is also introduced independently to the bottom of the riserthrough line 22 to help carry upwardly into the riser regeneratedcatalyst which flows to the bottom of the riser through transfer line26.

The high molecular weight hydrocarbon fraction is preheated to atemperature of about 600 F. in preheater 20 and is introduced throughline 24 into the upper section of the riser at the zone wherein thediameter of the riser becomes enlarged. The high molecular weighthydrocarbon charge is introduced at about a 45 upward angle into theriser through lines 30 and 32. Steam can be introduced into the highmolecular weight hydrocarbon inlet lines through lines 34 and 36. Highmolecular weight hydrocarbon lines 30 and 32 each represent a pluralityof similar lines spaced circumferentially at the same height of theriser. Any recycle hydrocarbon can be admitted to the upper section ofthe riser through one of the upwardly inclined inlet lines designated as38. No catalyst is added directly to the upper section of a riser butall of the catalyst is added at the bottom of the riser together withthe low molecular weight hydrocarbon feed. The residence times of boththe high molecular weight feed and the low molecular weight feed can bevaried by varying either the relative amounts or positions ofintroduction of the high and low molecular weight feed streams.Therefore, the high molecular weight feed stream can be introduced,through line 30, or alternately through higher or lower lines 30A or30B, respectively.

The full range oil charge to be cracked in the riser is a gas oil havinga boiling range of about 430 to 1 l,00 F. As indicated above, beforebeing charged the gas oil is fractionated into a low molecular weightfraction which is charged to the bottom of the riser and a highmolecular weight fraction which is charged to the top of the riser. Thesteam added to the riser amounts to about 10 weight percent based on theoil charge, but the amount of steam can vary widely. The steam is addedwith both the low and high molecular weight hydrocarbon fractions. Thecatalyst employed is a fluidized zeolitic aluminosilicate and is addedto the bottom only of the riser. Thc riser temperature range is about900 to l, l00 F. and is controlled by measuring the temperature of theproduct from the risers and then adjusting the opening of valve 40 bymeans of temperature controller 42 which regulates the inflow of hotregenerated catalyst to the bottom of the riser. The temperature of theregenerator catalyst is above the control temperature in the riser sothat the incoming catalyst contributes heat to the cracking reaction.The riser pressure is between about 10 and 35 p.s.i.g. Between about andpercent of the oil charge to the riser is normally recycled.

The residence time of both hydrocarbon and catalyst in the riser is verysmall and ranges from 0.5 to 5 seconds. The lower molecular weighthydrocarbon is usually in the riser for about two seconds because it isintroduced to the bottom of the riser but the higher molecular weighthydrocarbon will generally be in the riser for no more than about onesecond because it is introduced into the top of the riser. The velocitythroughout the riser is about 35 to 55 feet per second and issufficiently high so that there is little or no slippage between thehydrocarbon and catalyst flowing through the riser. Therefore, no bed ofcatalyst is permitted to build up within the riser, whereby the densitywithin the riser is very low. The density within the riser is a maximumof about 4 pounds per cubic foot at the bottom of the riser anddecreases to about 2 pounds per cubic foot at the top of the riser.Since no dense bed of catalyst is permitted to build up within the riserthe space velocity through the riser is usually high and will have arange between 100 or 120 and 600 weight of hydrocarbon per hour perinstantaneous weight of catalyst in the reactor. No significant catalystbuild up within the reactor is permitted to occur and the instantaneouscatalyst inventory within the riser is due to a flowing catalyst to oilweight ratio between about 4:] and l5:l, the weight ratio correspondingto the feed ratio.

The hydrocarbon and catalyst exiting from the top of each riser ispassed into a disengaging vessel 44. The top of the riser is capped at46so that discharge occurs through lateral slots 50 for properdispersion. An instantaneous separation between hydrocarbon and catalystoccurs in the disengaging vessel. The hydrocarbon which separates fromthe catalyst is primarily gasoline together with some heavier componentsand some lighter gaseous components. The hydrocarbon effluent passesthrough cyclone system 54 to separate catalyst fines contained thereinand is discharged to a fractionator through line 56. The catalystseparated from hydrocarbon in disengager 44 immediately drops below theoutlets of the riser so that there is no catalyst level in thedisengager but only in a lower stripper section 58. Steam is introducedinto catalyst stripper section 58 through sparger 60 to remove anyentrained hydrocarbon in the catalyst.

Catalyst leaving stripper 58 passes through transfer line 62 to aregenerator 64. This catalyst contains carbon deposits which tend tolower its cracking activity and as much carbon as possible must beburned from the surface of the catalyst. This burning is accomplished byintroduction to the regenerator through line 66 of approximately thestoichiometrically required amount of air for combustion of the carbondeposits. The catalyst from the stripper enters the bottom section ofthe regenerator in a radial and downward direction through transfer line62. Flue gas leaving the dense catalyst bed in regenerator 64 flowsthrough cyclones 72 wherein catalyst fines are separated from flue gaspermitting the flue gas to leave the regenerator through line 74 andpass through a turbine 76 before leaving for a waste heat boiler whereinany carbon monoxide contained in the flue gas is burned to carbondioxide to accomplish heat recovery. Turbine 76 compresses atmosphericair in air compressor 78 and this air is charged to the bottom of theregenerator through line 66.

The temperature throughout the dense catalyst bed in the regenerator isabout l,250 F. The temperature of the flue gas leaving the top of thecatalyst bed in the regenerator can rise due to afterbuming of carbonmonoxide to carbon dioxide. Approximately a stoichiometric amount ofoxygen is charged to the regenerator and the reason for this is tominimize afterburning of carbon monoxide to carbon dioxide above thecatalyst bed to avoid injury to the equipment since at the temperatureof the regenerator flue gas some afterburning does occur. In ordertoprevent excessively high temperatures in the regenerator flue gas due toafterbuming, the temperature of the regenerator flue gas is controlledby measuring the temperature of the flue gas entering the cyclones andthen venting some of the pressurized air otherwise destined to becharged to the bottom of the regenerator through vent line in responseto this measurement. The regenerator reduces the carbon content of thecatalyst from 1:0.5 weight percent to 0.2 weight percent, or less. ifrequired, steam is available through line 82 for cooling theregenerator. Makeup catalyst is added to the bottom of the regeneratorthrough line 84. Hopper 86 is disposed at the bottom of the regeneratorfor receiving regenerated catalyst to be passed to the bottom of thereactor riser through transfer line 26.

We claim:

1. In a process for cracking at least one relatively low molecularweight gas oil hydrocarbon feed stream and at least one relatively highmolecular weight gas oil hydrocarbon feed stream to gasoline in thepresence of a fluid zeolite cracking catalyst the improvement comprisingcharging the relatively low molecular weight hydrocarbon feed stream tosaid process at a relatively upstream position and charging therelatively high molecular weight feed steam to said process at arelatively downstream position along the reaction path, performing saidprocess at a temperature between 900 and l,l00 F. and a residence timeof less than five seconds during which catalyst and hydrocarbon bothflow concurrently through the process under conditions such as to avoidformation of a catalyst bed in the reaction flow stream, the cracking ofsaid low molecular weight hydrocarbon feed stream performed in thepresence of an added diluent vapor which reduces the partial pressure ofsaid low molecular weight hydrocarbon feed and produces a net increasein debutanized gasoline yield in said process, and recoveringdebutanized gasoline from said process in an amount including said netincrease.

2. The process of claim I wherein the reactor is enlarged near theposition of introduction of said high molecular weight feed stream sothat the linear velocity before and after the enlargement is betweenabout 25 and 75 feet per second and the total reactor length to diameterratio is above about 20.

3. The process of claim I wherein the catalyst to low molecular weighthydrocarbon weight ratio is between about 4:1 toabout 15:1.

4. The process of claim 1 wherein the density of the material at the lowmolecular weight feed inlet is about 1 to 4.5 pounds per cubic foot.

5. The process of claim I wherein said diluent vapor is steam and ispresent in an amount between about 0.5 to 10 weight percent based on thelow molecular weight feed.

6. The process of claim I wherein said catalyst is charged to theprocess at a temperature of at least about L240 F.

7. The process of claim I wherein the diluent vapor is steam, nitrogen,methane or ethylene.

8. The process of claim 1 wherein the effluent stream discharges fromthe cracking reactor in a lateral direction.

9. The process of claim 1 wherein said low molecular weight feed streamand said high molecular weight feed stream are fractions of a common gasoil hydrocarbon stream.

10. The process of claim 1 wherein the pressure is about 5 to 50 oundsper square inch gauge.

11. he process of claim 1 wherein the space velocity based upon all feedstreams is at least about weight of hydrocarbon feed per hour per weightof catalyst.

* 4 i I i November Dated Patent No. 3 617 497 Millard C. Bryson andJames R. Murphy the above-identifies: patent 3; untreated as shcwn'belsw:

It is certified that errcr appears in that aaid Letters Patent are here:

Invan'wfls) and COLUMN 8, LINE 3G, DELETE "15 A! INSERT --1U-- INSERTUNDER LINE 37 DELETE "Butenes 11.6" AND ---Butenes-- UNDER "n-Eutane"INSERT --'i.l.u-- 182 11! COLUMN l1 COLUMN 12 LIIMJ 10, IN COLUMN 1 AND3, DELETE (1) AND INSERT ---{2) and seale this: 231%: day of 2* y W72

2. The process of claim 1 wherein the reactor is enlarged near theposition of introduction of said high molecular weight feed stream sothat the linear velocity before and after the enlargement is betweenabout 25 and 75 feet per second and the total reactor length to diameterratio is above about
 20. 3. The process of claim 1 wherein the catalystto low molecular weight hydrocarbon weight ratio is between about 4:1 toabout 15:
 4. The process of claim 1 wherein the density of the materialat the low molecular weight feed inlet is about 1 to 4.5 pounds percubic foot.
 5. The process of claim 1 wherein said diluent vapor issteam and is present in an amount between about 0.5 to 10 weight percentbased on the low molecular weight feed.
 6. The process of claim 1wherein said catalyst is charged to the process at a temperature of atleast about 1,240* F.
 7. The process of claim 1 wherein the diluentvapor is steam, nitrogen, methane or ethylene.
 8. The process of claim 1wherein the effluent stream discharges from the cracking reactor in alateral direction.
 9. The process of claim 1 wherein said low molecularweight feed stream and said high molecular weight feed stream arefractions of a common gas oil hydrocarbon stream.
 10. The process ofclaim 1 wherein the pressure is about 5 to 50 pounds per square inchgauge.
 11. The process of claim 1 wherein the space velocity based uponall feed streams is at least about 100 weight of hydrocarbon feed perhour per weight of catalyst.